MutableFileNode.set_downloader_hints: never depend upon order of dict.values()

The old code was calculating the "extension parameters" (a list) from the
downloader hints (a dictionary) with hints.values(), which is not stable, and
would result in corrupted filecaps (with the 'k' and 'segsize' hints
occasionally swapped). The new code always uses [k,segsize].

src/allmydata/mutable/filenode.py

@@ -701,7 +701,7 @@ class MutableFileNode:
 
     def set_downloader_hints(self, hints):
         self._downloader_hints = hints
-        extensions = hints.values()
+        extensions = [ hints["k"], hints["segsize"] ]
         self._uri.set_extension_params(extensions)